It started with a plume – early Archaean basaltic proto-continental crust

Abstract

The lower part of the Pilbara Supergroup records 300 million years of voluminous basaltic magmatism from ca. 3.515 to 3.24 Ga. The basalts are divided into two compositionally distinct but contemporaneous and interbedded types. Compared to low-Ti basalts, high-Ti basalts (TiO 2 > 0.8 wt.%) have relatively high concentrations of HFSE and REE, are generally more Fe-rich, have very low Al 2O 3/TiO 2 (18.7–8.9) and high Gd / Yb ratios (1.12–2.23). The composition of the high-Ti basalts, and their source, did not change significantly throughout the 300 m.y. period of basalt eruption. In contrast, low-Ti basalts show distinct secular trends to lower concentrations of incompatible trace elements and to lower ratios of La / Sm, La / Gd, La / Yb and Gd / Yb that reflect a source progressively more depleted than N-MORB source. Gd / Yb ratios in the younger low-Ti basalts are as low as 0.67, well below estimates for modern depleted mantle (0.98). These values reflect a strongly depleted source, and possibly also a contribution from a low Gd/Yb source component representing the garnet-bearing residue of previous high-pressure melting events. The source for the low-Ti basalts comprised the accumulated residues of plumes that produced the earliest high-Ti basalts and of plumes that produced subsequent high-Ti basalts, and was periodically modified through melt extraction. It remained isolated from the convecting asthenosphere throughout the 300 m.y. period of basaltic magmatism, but parts of it remelted each time one of a series of plumes impinged on the lithospheric mantle. It progressively formed the thick, depleted, and buoyant sub-continental lithospheric mantle (SCLM) that is complimentary to the thick pile of interbedded high- and low-Ti basalt that comprises the bulk of the Pilbara Supergroup. These interpretations support models for Palaeoarchaean protocrust formation through extensive mantle plume magmatic events, and models that suggest that the Archaean SCLM formed at the same time as early voluminous mafic magmatism.